var VSHADER_SOURCE =
    `attribute vec4 a_Position;
    attribute vec4 a_Color;
    varying vec4 v_Color;
    void main() {
      gl_Position = a_Position;
      v_Color = a_Color;
    }`;

// Fragment shader program
var FSHADER_SOURCE =
    `precision mediump float;
    varying vec4 v_Color;
    void main() {
      gl_FragColor = v_Color;
    }`;

function main() {
    // Retrieve <canvas> element
    var canvas = document.getElementById('webgl');

    // Get the rendering context for WebGL
    var gl = getWebGLContext(canvas);
    if (!gl) {
        console.log('Failed to get the rendering context for WebGL');
        return;
    }

    // Initialize shaders
    if (!initShaders(gl, VSHADER_SOURCE, FSHADER_SOURCE)) {
        console.log('Failed to intialize shaders.');
        return;
    }

    // 
    var n = initVertexBuffers(gl);
    if (n < 0) {
        console.log('Failed to set the vertex information');
        return;
    }

    // Specify the color for clearing <canvas>
    gl.clearColor(0.0, 0.0, 0.0, 1.0);

    // Clear <canvas>
    gl.clear(gl.COLOR_BUFFER_BIT);

    // Draw three points
    gl.drawArrays(gl.TRIANGLES, 0, n);
}

function initVertexBuffers(gl) {
    var verticesColors = new Float32Array([
        // Vertex coordinates and color
        0.0, 0.5, 1.0, 0.0, 0.0, -0.5, -0.5, 0.0, 1.0, 0.0,
        0.5, -0.5, 0.0, 0.0, 1.0,
    ]);
    var n = 3; // The number of vertices

    // Create a buffer object
    var vertexColorBuffer = gl.createBuffer();
    if (!vertexColorBuffer) {
        console.log('Failed to create the buffer object');
        return false;
    }

    // Write the vertex coordinates and colors to the buffer object
    gl.bindBuffer(gl.ARRAY_BUFFER, vertexColorBuffer);
    gl.bufferData(gl.ARRAY_BUFFER, verticesColors, gl.STATIC_DRAW);

    var FSIZE = verticesColors.BYTES_PER_ELEMENT;
    //Get the storage location of a_Position, assign and enable buffer
    var a_Position = gl.getAttribLocation(gl.program, 'a_Position');
    if (a_Position < 0) {
        console.log('Failed to get the storage location of a_Position');
        return -1;
    }
    gl.vertexAttribPointer(a_Position, 2, gl.FLOAT, false, FSIZE * 5, 0);
    gl.enableVertexAttribArray(a_Position); // Enable the assignment of the buffer object

    // Get the storage location of a_Position, assign buffer and enable
    var a_Color = gl.getAttribLocation(gl.program, 'a_Color');
    if (a_Color < 0) {
        console.log('Failed to get the storage location of a_Color');
        return -1;
    }
    gl.vertexAttribPointer(a_Color, 3, gl.FLOAT, false, FSIZE * 5, FSIZE * 2);
    gl.enableVertexAttribArray(a_Color); // Enable the assignment of the buffer object

    return n;
}